This invention relates to a hydraulic control system and particularly to one used as a shot control system for a die-casting machine.
In the process of die-casting, molten metal such as zinc, aluminum or magnesium is forced under pressure into a closed metal die. Although the die-casting process is well understood efforts are continually being directed toward optimizing the process. In particular, the process of controlling the "shot" or the forcing of molten metal into an empty die has undergone a great deal of study. The shot process has a direct influence on final part quality. Scrap and quality degradation occur when the final part has porosity or other variations in physical properties caused by a lack of precise control over the shot process. In addition, there is a desire to reduce the flash or excess metal which remains on the part at the die parting line after it is ejected from the dies.
In an effort to improve die cast quality, manufacturers of die casting equipment have attempted to provide more precise control over the shot process. A typical die-caster has a hydraulic shot cylinder oriented either horizontally or vertically which drives a plunger which forces a volume of molten metal into the die cavity under pressure. In some machines, the dies are preheated and thus referred to as "hot chamber" machines as compared with unheated "cold chamber" machines. Efforts have centered on precisely controlling the velocity of the shot plunger at various points along its stroke. Although great strides have been made in the shot control process through the use of sophisticated servo hydraulic control systems, presently available shot control systems do not provide a repeatable shot process. Advanced systems currently available are described as closed-loop controllers but lack the characteristics necessary to provide precise repeatable control. Disadvantages of currently available shot control systems primarily relate to two areas. First, today's systems lack the frequency response necessary to enable the system to rapidly respond to differences between a desired shot cylinder plunger velocity at a particular point in its travel, and its actual velocity at that point. The limited frequency response of currently available systems also prevents the system from quickly recognizing the occurrence of the die being fully filled with molten metal which, when driven further, creates a "water hammer" effect in which a high pressure shock is applied to the casting dies. This shock causes deflection of the dies which produces excess flash on the cast part.
A second disadvantage of current systems is their degradation over time caused by impurities which are invariably present in hydraulic fluid. Conventional hydraulic servo valves either of the jet pipe or nozzle-flapper variety which are the two most popular types, are highly sensitive to contaminants. Present systems therefore have stringent hydraulic fluid filtration requirements often in the 3 to 5 micron range. Currently used shot control servo valves typically must be re-built on a very frequent basis to maintain adequate shot control. Moreover, even during the operating cycle of currently available servo valves, between servicing a gradual degradation of performance occurs between valve rebuilds. This degradation is noticed as a decrease in the valves slew rate or frequency response. Conventional jet pipe and nozzle flapper servo valves are further two stage devices which inhibits their frequency response capabilities.
In accordance with this invention, an improved die caster hydraulic shot control system is provided having a number of novel features, which when combined, produce a significant improvement over existing shot control systems. The hydraulic shot control system in accordance with this invention features a voice coil driven pilot servo valve which inherently features high frequency response. In addition, the voice coil driven pilot servo valve provides high flow capabilities which eliminates multi-stage servo valves normally required in currently available systems, further enhancing frequency response. The pilot servo of this invention is inherently contaminant tolerant which is a significant advantage in a production plant setting. The use of a pilot servo valve of this configuration coupled with a high flow shot proportional valve along with additional features combine to provide exceptional control, repeatability, stability and low maintenance in the shot control process.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, conjunction with the accompanying drawings.